Spaceborne sun pumped laser

ABSTRACT

An improved sun pumped laser communication system for synchronous satellites in the galactic plane is provided by mounting the sun pumped laser in a sun tracking telescope pivoted in hollow gimbals placed on the axis of rotation of the satellite. The laser beam is directed to the earth or another spacecraft receiver from the satellite by a tracking telescope also pivoted in hollow gimbals placed on the axis of rotation of the satellite. The laser beam traverses the satellite along its axis of rotation by passing through the hollow gimbals. The polarization of the laser beam is adjusted for maximum efficiency and controlled to compensate for the orientation changes between the laser and modulator. By mounting the laser in the sun tracking telescope with the laser heat sink communicating with a thermally emissive face in the side of the telescope tube, the laser is cooled by thermal radiation which is always automatically directed toward deep space.

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States atent Barry et a1.

Maiman 33 l/94.5 Freedman 4. 331/945 5 7 ABSTRACT An improved sun pumpedlaser communication system for synchronous satellites in the galacticplane is provided by mounting the sun pumped laser in a sun trackingtelescope pivoted in hollow gimbals placed on the axis of rotation ofthe satellite. The laser beam is directed to the earth or anotherspacecraft receiver from the satellite by a tracking telescope alsopivoted in hollow gimbals placed on the axis of rotation of thesatellite. The laser beam traverses the satellite along its axis ofrotation by passing through the hollow gimbals. The polarization of thelaser beam is adjusted for maximum efficiency and controlled tocompensate for the orientation changes between the laser and modulator.By mounting the laser in the sun tracking telescope with the laser heatsink communicating with a thermally emissive face in the side of thetelescope tube, the laser is cooled by thermal radiation which is alwaysautomatically directed toward deep space.

6 Claims, 7 Drawing Figures SPACEBORNE SUN PUMPED LASER [75] Inventors:James D. Barry, Fairborn; Paul M.

Freedman, Wright-Patterson AFB; George Matassov, Dayton, all of Ohio[73] Assignee: The United States of America as represented by theSecretary of the United States Air Force, Washington, DC.

[22] Filed: May 1, 1973 [21] App]. No.: 356,270

Related US. Application Data [63] Continuation-in-part of Ser. No.340,515, March 12,

[52] US. Cl. 331/945 [51] Int. Cl. H0ls 3/02 [58] Field of Search331/945; 244/1 SS, 244/1 SA [56] References Cited UNITED STATES PATENTS3,297,958 1/1967 Weiner 331/945 3,421,715 l/1969 Cohlan 331/945 Sll/VR096 PATENIEHJAM 15 m4 sum 0F 5 HEHT sysren SPACEBORNE SUN PUMPED LASERRELATED APPLICATION This is a continuation-in-Part of prior patentapplication Ser. No. 340,515, filed Mar. 12, 1973.

BACKGROUND OF THE INVENTION The field of the invention is in thesatellite communication art.

Solar (sun pumped) lasers are well known. US. Pat. No. 3,297,958 topatentee M. Weiner is an example of an end pumped solar laser and US.Pat. No. 3,451,010 to patentee T. H. Maiman shows an example of a sidepumped solar laser. Technical Report AD481927 Sun-Pumped Laser by C. G.Yound (1966), and Technical Report AD-737787, Sun Pumped Laser by LloydHuff (1971), are also examples of the prior art.

In the prior art satellite laser communication systems the laser hasbeen located in the body of the satellite and the cooling of the laserrod has been a problem.

The excess heat energy has to be removed from the laser by a coolingsystem within the satellite, directed by various paths through thesatellite, and radiated to space. For effective radiation to take placethe thermal radiation must be directed in the direction of deep space,that is, at right angles to the galactic plane. This has necessitated avery complicated thermal system because the satellite has its spin axisalso perpendicular to the galactic plane and its attitude in space isgenerally constantly slowly changing. Some prior art devices havecomplicated optical systems for directing the suns rays onto the laser.The routing of the laser energy from the laser to the pointingtelescope, directed toward the distant receiver, has also requiredcomplicated optical universal joints to direct the laser beam throughthe satellite and compensate for the many angular variations.

SUMMARY OF THE INVENTION The invention provides a sun pumped lasercommunication system for satellites wherein the laser element is easilycooled by direct radiation into deep space and the polarization of thelaser beam is constantly maintained at the appropriate angle withrespect to the modulator for all angular variations of the polarizationdue to the directing optics. The laser beam is directed from the lasersource through the satellite to the transmitting telescope andassociated optical elements along the satellite axis by using hollowgimbals.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a pictorial view of asatellite in space incorporating the invention;

FIG. 2 is a schematic diagram of an embodiment of the invention having acontrolled polarization rotation crystal;

FIG. 3 is a schematic diagram of typical hollow gimbals;

FIG. 4 is a schematic diagram of a representative sun tracking telescopehaving Cassegrain type optics;

FIG. 5 is a schematic diagram of a representative sun tracking telescopehaving a light concentrating cone;

FIG. 6 is a schematic diagram of an embodiment of the invention having.polarization control by quarter wavelength plates; and

FIG. 7 is a schematic diagram of typical hollow gimbals having a quarterwavelength polarization changing element.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a pictorial view ofa representative satellite spacecraft lll having the disclosedinvention. Generally the spacecraft is a synchronous, geostationarysatellite rotating about an axis aligned with the axis of the earth suchthat the rotation axis is substantially perpendicular to the galacticplane. The general configurations of such spacecraft are that of acylinder with the axis of the cylinder coinciding with the axis ofrotation. The ends of the cylinder, being substantially perpendicular tothe axis of rotation, do not have the sun in view along the axis. Inorder to place the solar energy collector (that is the sun trackingtelescope 13), and the laser transmitter pointing optics (that is theearth (or another satellite) tracking telescope 14), in positions ofmutual non-interference, as well as being directionally independent ofthe synchronous spacecraft, they are placed at opposite ends of thecylindrical spacecraft and pivoted in gimbals on the spacecraft axis ofrotation. Each telescope has its own independent pointing system drivingthrough its respective gimbals. By mounting the laser in the suntracking telescope 13, which is activated to continuously point towardthe sun, the cooling of the laser may readily be accomplished by havinga radiating surface 15 (which is thermally connected with the laser heatsink) in the side of the telescope. Thus, the thermal radiator isautomatically and without any mechanical complications always lookinginto deep space independent of the orientation of the spacecraft.

The optical path of the laser beam is directed through the hollowgimbals l6 and 17, and substantially along the spin axis of thespacecraft while traversing the spacecraft. This arrangement provides alaser system that is relatively independent of the spacecra'fts attitudeand rotation. It also eliminates the need for complicated internaldirecting optics.

A detailed schematic diagram of an embodiment of the invention is shownin FIG. 2. The sun tracking telescope 13 has conventional Cassegrainoptics with primary collector mirror 21 and secondary collector mirror22 concentrating the solar energy on the end of conventional end pumpedlaser rod 23. The laser has the conventional laser optics 24 and 25, andelectronic system 26. The laser is maintained at 0C or below by aconventional heat sink that surrounds the laser rod and is thermallyconnected to the conventional blackbody thermal radiator 15 radiatingthermally directly into deep space. It is to be noted that with the suntracking telescope pointing at the sun that the radiation from thethermal radiator will always be automatically directed to deep space.The area of the thermal radiator which is mounted on the sun trackingtelescope is determined bythe amount of thermal power to be dissi pated.For example in a specific embodiment where 20 watts of thermal energyare to be dissipated to maintain the laser rod at a temperature of 0C anarea of 500 cm provided the required cooling. (With a deep spacetemperature of about 3K.) Those persons practicing this invention willreadily adapt the area of the black-body deep space radiator toaccommodate the dissipation requirements of the particular laser beingused. (Obviously, other separate conventional thermal radiators may beused in the satellite to provide the necessary cooling of other on-boardequipment.) Additional details of embodiments of the sun trackingtelescope will be discribed later in connection with FIGS. 4 and 5.

The laser beam 27 from the laser is directed into the satellite body 11along its axis of rotation 12 by the folding mirror 28. (Also see FIG.3.) It is generally desirable that the laser beam be plane polarized toa high degree and that the orientation changes caused by reflectionsfrom the directing optics not interfere with the beam modulation.Polarization orientation modifications after beam modulation do notinterfere with data transmission. Mirror 28 is referred to as a foldingmirror in that its magnitude of angle of rotation is one-half that ofthe angle of rotation of the telescope. This also applies to the foldingmirror 29 in the receiver tracking telescope 14. Conventional laser beammodulator 30 modulates the beam in accord with the intelligencecontained in the signal being transmitted to the earth or otherappropriate remote receiver. An example of a suitable beam modulatorelement is the commercially available lithium tantalate electro-opticalcrystal modulator.

Polarization rotation crystal 31 is a conventional electro-opticalpolarization rotation crystal. The plane of polarization of the laserbeam is electro-optically rotated in response to the electric fieldbetween the electrodes on the crystal. The electric field across thecrystal is controlled by the onboard computer 32. A change in thevoltage potential between the electrodes on the crystal changes theelectric field across the crystal and hence its amount of rotation ofthe polarization of the beam. The polarization rotation crystal 31,which may be a conventional lithium tantalate polarization rotationcrystal, adjusts the plane of polarization of the laser beam to presenta fixed plane of polarization of the laser beam to the modulator 30.This makes the modulation changes in the polarization of the beambrought about by the modulation crystal a true representation of theintelligence transmitted, uneffected by polarization changes takingplace in the laser beam prior to the modulator. The reflection of thelaser light by the folding mirror 29 causes well known modifications tothe polarization orientation of the laser beam. The polarizationorientation of the laser beam at the input to crystal 3] also changeswith changes in the relative angular position of the sun trackingtelescope 13 with respect to the modulator. These changes and theinstant relative positions are all contained in the computer informationin the computer 32, and the computer supplies the voltage potential tothe electrodes on the polarization rotation crystal 31 such that thelaser beam entering the modulator 30 is at all times focused andcritically aligned to the preferred entrance polarization of themodulation crystal.

The computer and actuation system 2 is conventional with satellitespacecraft. It receives position indication and control signal inputsand provides control signals for station keeping, positioning, attitudechanging, antenna directing, switching and other on-board functions. Inthis invention the computer 32, in addition to these other functionscontrols the angle of polarization of the laser beam entering themodulator by sensing the angular orientation of the sun trackingtelescope 13, the folding mirror 29, and the preferred modulator axisand determines the appropriate voltage to crystal 31 so that the properpolarization of the laser beam is always presented to the modulator 30.It also controls the aiming of the sun trackingtelescope and thereceiver tracking telescope. The receiver telescope is directed towardthe earth or other spacecraft receiver by the computer from the relativepositioning information maintained in the computer in regard to theposition of the satellite with respect to the earth and by signalinformation transmitted to the satellite from the earth or otherreceiver station. The sun tracking telescope contains a conventional sunseeker element 33, positioned in axial alignment with the axis of thetelescope, that provides an electrical signal indicative of its pointingaccuracy toward the sun, that is, its accuracy of axial alignment withthe sun rays. The output from the sun seeker is conducted through twosets of slip rings in the gimbals 16 to the computer. The computer anddrive system 32 then generate the signals that actuate the drivemechanisms to point the telescope 13 toward the sun in accord with thesignals from the sun seeker element. Sun seeker and light directiondetecting devices are well known. An example of a suitable sun seekerelement is the Digital Solar Aspect Sensor manufactured by the AdeoleCorporation. Generally, the sun seeker element and the secondarycollector element 22 are axially mounted near the sun end of thetelescope on a common spider" support member.

FIG. 3 is a representative schematic diagram of an embodiment of thegimbals for the sun tracking telescope. The folding mirror 28 is locatedsuch that the center of the mirror is always maintained at theintersection of the spacecraft rotation axis 12 and the axis of thetelescope 35. Servomotor 36 rotates the outer member of the gimbalsabout the spacecraft rotation axis and servomotor 37 positions thetelescope to the desired angle of tilt, that is, the angle between thetelescope axis and the rotation axis of the spacecraft. Both servomotorscomprise a part of the loop of the servo system positioning the sunseeker and hence the sun tracking telescope in alignment with the sunrays. In the hollow gimbals attaching the sun tracking telescope to thesatellite body two sets of slip ring assemblies 38 and 39 are necessary.The first set 38 carries the electrical signals going to the tilt drivemechanism 37 located in the first ring of the gimbals and also thosegoing through the second set of slip rings 39 into the second or innerring of the gimbals and onto the laser and sun seeker.

It has previously been mentioned that the folding mirror 28 traverses inangle one-half the angle traversed by the telescope 13 to alwaysmaintain the proper reflection of the laser beam from the axis of thetelescope 35 to the rotation axis of the satellite 12. This isaccomplished by conventional 2 to 1 gear boxes 40 and 41. (Direct drivetorque motors may also be used.) While only one gear box is required toprovide the necessary drive action, generally slightly better alignmentaccuracy is obtained using two, one at each side of the folding mirror.The shaft attached to the mirror 28 turns on bearing surfaces inside thelarger hollow shaft attached to the telescope 13. The inner shaft isdriven in rotation by the outer shaft through the planetary reductiongearing in the gear boxes 40 and 41. The gimbals attaching the receivertracking telescope to the satellite are identical in configuration withthose described for the sun tracking telescope except slip ring assembly39 is not needed since for this invention the earth tracking telescopedoes not contain electrical signal generating or utilizing elements.

FIGS. 4 and 5 show schematically two representative embodiments of sunpumped laser configurations. The means for actuating the sun trackingtelescopes 45 and 55, including the sun seeker elements 46 and 56, areas previously disclosed. Likewise, the directing of the laser beams 47and 57 from the telescope through the satellite and onto the earth orother spacecraft are as previously disclosed. The optical system shownin FIG. 4, comprising the primary collector mirror element 48, thesecondary collector reflecting element 49, and the focusing lens element50, is the conventional Cassegrain type of structure. The secondarycollector 49 has on the laser side an optical surface which is highly(effectively 99+ percent) reflective at the laser pumping frequency,such as, within the 7,300A to 9,000A band and transmissive to otherwavelengths. The other side of element 49 is highly reflective to allwavelengths. By using such optical surfaces the useless solar energytransferred into the laser system is substantially reduced therebyreducing the thermal load on the laser system to be dissipated. Suchcoatings are commercially available and well known in the cold mirrorart. The laser system comprising the solid state laser rod element 51,the laser mirror elements 52 and 53, and the mode locking or singlefrequency element 54 (if used), are all conventional elements. Theirstructure and the techniques of their use are all well known. The laserrod 51 is surrounded by the heat sink 55. In this particular embodimentheat-pipe structure 56 is used to transfer the heat energy from the heatsink 55 to the conventional black body thermal radiator 57. In someembodiments, depending on the particular laser used and the amount ofheat to be removed from the laser and dissipated, the heat pipestructure 56 may not be needed and direct thermal contact from the laserheat sink 55 to the radiator 57 may be used as shown in the embodimentshown in FIG. 5.

In the embodiment illustrated schematically in FIG. 5 the solarradiation is concentrated in the laser rod 60 by the cone field lens 61and the light concentrating cone 62. The laser optical cavity totallyreflecting mirror 63, defining one end of the optical cavity, ispositioned on the back side of the cone field lens in this embodimentinstead of on the end of the laser rod as in the embodiment of FIG. 4.(The mirror 52, of FIG. 4, is reflective to the laser light frequencyand substantially transmissive to pumping frequency of the sun light.)Mode locking or single frequency element 64 and laser output mirror 65are similar to the previously described embodiment. This laser structureis well known in the art and further described in the previouslymentioned prior art referenced publication AD-737787. The heat sink 66transfers the dissipated heat energy from the laser rod to the deepspace radiating surface 67 by direct thermal conduction.

One particular embodiment of the invention has a Nd:YAG laser rodelement. Approximately 400 watts of incident solar energy is availableat the effective optical antenna, that is, the opening at the sun end ofthe sun tracking telescope which is about 0.32 meter in diameter. Theuseful laser output from the system is approximately 1 watt.Approximately watts of thermal power was dissipated and radiated to deepspace. The laser was maintained at approximately a temperature of 0C.The laser mirror at the sun end of the laser was highly reflective atsubstantially. 1.06 microns, (the laser frequency) and highlytransmissive in the pump bands of 7,300A to 9,000A. Selective reflectiveand antireflective coatings are well known as is their use in laseroptical cavities. Generally the pump cavity surfaces should be highlyreflective at the pump bands (such as 7,300A to 10,000A) with a wideenough bandwidth to allow for reflection at various angles of incidence,and transmissive at all other wavelengths so that unwanted radiationenergy may be deposited in the substrait structure and removed as heat.

Generally, synchronous satellites are nominally in the galactic planeand stabilized such that the spin axis of the satellite spacecraft issubstantially constantly perpendicular to the galactic plane. Inaddition the satellite spin may be made to be one revolution per year,i.e., a solar phased satellite. Under these conditions the tracking ofthe sun and the movements relative to the spacecraft of the sun trackingtelescope are very small. Also, the movements of the folding mirror arethus very small. For instance, referring to FIG. 6, the sun trackingangle, that is the angle between the axis 35 of the sun trackingtelescope tube 113 and the spacecraft rotation axis 12, is substantiallySince the spin axis is stabilized within a few degrees the variations inthe sun tracking angle from 90 will generally be within i 2. Thus, underthese conditions the angular variations of the folding mirror in the suntracking telescope from 45 will be approximately i 1. These smallvariations greatly reduce the reflection losses occurring to the beamupon reflection from the folding mirror for a spe- CiflC polarizationand beam incidence angle.

The laser beam output from the conventional laser 23 is a planepolarized beam. The plane of the polarization is basically dependentupon the physical rotational orientation of the laser structure,however, it may also be controlled by intercavity elements irrespectiveof the particular positioning of the physical structure. The effects onthe polarization of a plane polarized wave by reflecting from dielectricand metallic mirrors are well known. (See Born and Wolf, Principles ofOptics," second edition, commencing at page 619, and Jenkins and White,Fundamentals of Optics, third edition, commencing at pages 515 and 520.)

The embodiment of the invention shown schematically in FIGS. 6 and 7 issomewhat simplified in construction from the embodiment shown in FIGS. 2and 3 in that no computer controlled polarization rotation crystal isrequired. However, a limitation is required on the relationship of theplane of polarization of the laser beam and the rotation axis of thefolding mirror in the sun tracking telescope, and two quarter wavelengthplates are used instead of the computer controlled polarization rotationcrystal. Referring to FIG. 6, the laser 23 is arranged such that theplane of polarization of the laser beam striking the folding mirror 28is either perpendicular or parallel to the mirror axis of rotation 80.That is, the laser beam striking the folding mirror is plane polarizedin a direction perpendicular to the plane formed by the mirror normaland the axis of incidence, or in a direction parallel to the axis 12 andwithin that plane. This will avoid the possibility of the formation ofelliptically polarized light in the reflection from the mirror. Theperpendicular component is generally preferred as it is approximately 2percent more reflective than the parallel component. This is generallythe situation with metal mirrors and it is also true for dielectriccoatings at approximately 45 incidence. The phase of the reflected beammay change with respect to the incident beam with various types ofmirrors used. The phase change is not critical; that the incident beamis perpendicular or parallel plane polarized as it strikes the mirror iscritical, and as previously stated the perpendicularly polarized beam ispreferred. Generally a dielectric type mirror is preferred to a metalone since it may more readily be optimized for any particular laserwavelength used in the apparatus. These mirrors and coatings, whethermetal or dielectric, are well known and commercially available from manysources. Plane polarized light represented 81, FIG. 7, propagating alongtelescope tube axis 35 strikes mirror 28 with an angle of incidence ofapproximately 45, with the plane of polarization either parallel orperpendicular, but preferably parallel to mirror axis 80 as isrepresented 81.

With the plane of polarization of the laser beam either parallel orperpendicular to the mirror axis, the plane polarization characteristicof the beam is preserved as it reflects from the mirror. It is to benoted that the plane of polarization of the beam, with respect to thespacecraft body, as it leaves the mirror will rotate with the rotationof the gimbals about the rotation axis 12 of the spacecraft. Theconventional beam modulator 30 (FIG. 6) requires a predetermined fixed(stationary) plane polarized beam input. By using a conventional quarterwavelength (at the laser beam frequency) plate 82 that rotates with thegimbals about the spacecraft rotation axis to change the plane polarizedbeam coming from the folding mirror 28 to a circularly polarized beam(either right hand or left hand) and another quarter wavelength plate 83to change the circularly polarized beam back to a plane polarized beam,a constant position plane polarized beam is presented to the beammodulator 30. It is required, as is well known, that the quarter-waveplate 82 be physically positioned with respect to the plane polarizedbeam entering it such that the plane of polarization of theperpendicularly incident beam on it be at an angle of 45 with the opticaxis of the plate to convert the plane polarized beam to a circularlypolarized beam. The angle of the plane polarized beam emanating fromquarter-wave plate 83 is also dependent upon the physical positioning ofits optic axis. Thus, either quarterwave plate 83 or the beam modulatorelement 30, or both, are rotated with respect to each other to providethe proper relationship of the plane polarized beam entering themodulator. Commercially available modula tors may be specified toincorporate a quarter-wave plate properly positioned in their structureso they can receive a circularly polarized light input and provide amodulated plane polarized light output. In which case quarter-wave plate83, as a separate item, is not required.

It is generally preferred that the transposition from plane polarizedlight to circularly polarized take place after the laser beam has beenreflected from the folding mirror. However, quarter-wave plate 82changing the beam from plane polarized to circularly polarized may bepositioned in the sun tracking telescope following the laser and beforethe folding mirror. When this configuration of structure is used someloss occurs due to the slightly elliptical polarization resulting fromangular changes in the position of the folding mirror. The secondquater-wave plate produces a plane polarized wave from the ellipitcalwave but with slightly reduced energy content from that of the preferredstructure.

The beam modulator 30 of FIG. 6 is similar to the modulator of theprevious embodiment shown in FIG. 2. Laser beam modulators are wellknown and readily available commercially for modulation rates belowl00Mbps (Mega-bits per second) and available on special order up to1,000 Mbps. Any of the conventional laser beam modulation techniquesexternal to the laser may be used with the disclosed system such as,pulse gated binary modulation (PGBM), quadruphase shift key modulation(QPSK), and others. These (external modulators) are preferred so thatthe high speed modulators may be physically placed within the spacecraftbody rather attempting to transfer high speed electrical signals,requiring wide bandwidths, through the slip rings into the sun seekingtelescope with the attendant losses and data errors.

We claim:

1. A laser communication system for transmitting a signal from aspacecraft to a remote receiver, said spacecraft having an axis ofrotation, comprising:

a. a first set of hollow gimbals mounted on said spacecraft on the saidaxis of rotation;

b. a second set of hollow gimbals mounted on said spacecraft on the saidaxis of rotation in spaceapart relationship to the said first set ofgimbals;

c. a sun tracking telescope tube having a central axis attached to thesaid first set of gimbals;

d. a remote receiver tracking telescope attached to the said second setof gimbals;

e. a first folding mirror positioned in the said first set of gimbals,the said first folding mirror having an axis of rotation perpendicularto the spacecraft axis of rotation;

f. a second folding mirror positioned in the said second set of gimbals,the said second folding mirror having an axis of rotation perpendicularto the spacecraft axis of rotation;

g. a solar pumped laser receiving rays from the sun and generating aplane polarized laser beam along the said sun tracking telescope axis;

h. a heat sink cooperating with the said solar pumped laser, positionedin the said sun tracking telescope, radiating heat energy into deepspace;

i. means for positioning the said laser in the said sun trackingtelescope so that the polarization of the laser beam is parallel to thesaid axis of rotation of the said first folding mirror;

j. a first quarter-wavelength plate mounted in the said first gimbals,perpendicular to the spacecraft axis of rotation for changing the saidplane polarized laser beam to a substantially circularly polarized beam;

k. a second quarter-wavelength plate mounted in the said spacecraftperpendicular to the said spacecraft axis of rotation for changing thesaid circularly polarized beam to a plane polarized beam; and

1. means responsive to the said signal to be transmitted to the remotereceiver, cooperating with the said plane polarized beam from the saidsecond quarter-wavelength plate, for modulating the said laser beam.

2. The laser communication system as claimed in claim 1 wherein a sunseeker detector is positioned in the said sun tracking telescope andmeans cooperates with the said sun seeker for directing the sun trackingtelescope toward the sun.

3. The laser communication system as claimed in claim 2 wherein the saidsolar pumped laser is an end pumped laser.

4. A laser communication system for transmitting a signal from aspacecraft to a remote receiver, said spacecraft having an axis ofrotation, comprising:

a. a first set of hollow gimbals mounted on said spacecraft on the saidaxis of rotation;

b. a second set of hollow gimbals mounted on said spacecraft on the saidaxis of rotation in spacedapart relationship to the said first set ofgimbals;

c. a sun tracking telescope tube having a control axis attached to thesaid first set of gimbals;

d. a remote receiver tracking telescope attached to the said second setof gimbals;

e. a first folding mirror positioned in the said first set of gimbals,the said first folding mirror having an axis of rotation perpendicularto the spacecraft axis of rotation;

f. a second folding mirror positioned in the said second set of gimbals,the said second folding mirror having an axis of rotation perpendicularto the spacecraft axis of rotation;

g. a solar pumped laser receiving rays from the sun and generating aplane polarized laser beam along the said sun tracking telescope axis;

h. a heat sink cooperating with the said solar pumped laser, positionedin the said sun tracking telescope radiating heat energy into deepspace;

. means for positioning the said laser in the said sun trackingtelescope so that the polarization of the laser beam is perpendicular tothe said axis of rotation of the said first folding mirror andperpendicular to the said sun tracking telescope axis;

j. a first quarter-wavelength plate mounted in the said first gimbals,perpendicular to the spacecraft axis of rotation for changing the saidplane polarized laser beam to a substantially circularly polarized beam;

k. a second quarter-wavelength plate mounted in the said spacecraftperpendicular to the said spacecraft axis of rotation for changing thesaid circularly polarized beam to a plane polarized beam; and

1. means responsive to the said signal to be transmitted to the remotereceiver, cooperating with the said plane polarized beam from the saidsecond quarter-wavelength plate, for modulating the said laser beam.

5. The laser communication system as claimed in claim 4 wherein a sunseeker detector is positioned in the said sun tracking telescope andmeans cooperates with the said sun seeker for directing the sun trackingtelescope toward the sun.

6. The laser communication system as claimed in claim 5 wherein the saidsolar pumped laser is an end pumped laser.

1. A laser communication system for transmitting a signal from aspacecraft to a remote receiver, said spacecraft having an axis ofrotation, comprising: a. a first set of hollow gimbals mounted on saidspacecraft on the said axis of rotation; b. a second set of hollowgimbals mounted on said spacecraft on the said axis of rotation inspace-apart relationship to the said first set of gimbals; c. a suntracking telescope tube having a central axis attached to the said firstset of gimbals; d. a remote receiver tracking telescope attached to thesaid second set of gimbals; e. a first folding mirror positioned in thesaid first set of gimbals, the said first folding mirror having an axisof rotation perpendicular to the spacecraft axis of rotation; f. asecond folding mirror positioned in the said second set of gimbals, thesaid second folding mirror having an axis of rotation perpendicular tothe spacecraft axis of rotation; g. a solar pumped laser receiving raysfrom the sun and generating a plane polarized laser beam along the saidsun tracking telescope axis; h. a heat sink cooperating with the saidsolar pumped laser, positioned in the said sun tracking telescope,radiating heat energy into deep space; i. means for positioning the saidlaser in the said sun tracking telescope so that the polarization of thelaser beam is parallel to the said axis of rotation of the said firstfolding mirror; j. a first quarter-wavelength plate mounted in the saidfirst gimbals, perpendicular to the spacecraft axis of rotation forchanging the said plane polarized laser beam to a substantiallycircularly polarized beam; k. a second quarter-wavelength plate mountedin the said spacecraft perpendicular to the said spacecraft axis ofrotation for changing the said circularly polarized beam to a planepolarized beam; and l. means responsive to the said signal to betransmitted to the remote receiver, cooperating with the said planepolarized beam from the said second quarter-wavelength plate, formodulating the said laser beam.
 2. The laser communication system asclaimed in claim 1 wherein a sun seeker detector is positioned in thesaid sun tracking telescope and means cooperates with the said sunseeker for directing the sun tracking telescope toward the sun.
 3. Thelaser communication system as claimed in claim 2 wherein the said solarpumped laser is an end pumped laser.
 4. A laser communication system fortransmitting a signal from a spacecraft to a remote receiver, saidspacecraft having an axis of rotation, comprising: a. a first set ofhollow gimbals mounted on said spacecraft on the said axis of rotation;b. a second set of hollow gimbals mounted on said spacecraft on the saidaxis of rotation in spaced-apart relationship to the said first set ofgimbals; c. a sun tracking telescope tube having a control axis attachedto the said first set of gimbals; d. a remote receiver trackingtelescope attached to the said second set of gimbals; e. a first foldingmirror positioned in the said first set of gimbals, the said firstfolding mirror having an axis of rotation perpendicular to thespacecraft axis of rotation; f. a second folding mirror positioned inthe said second set of gimbals, the said second folding mirror having anaxis of rotation perpendicular to the spacecraft axis of rotation; g. asolar pumped laser receiving rays from the sun and generating a planepolarized laser beam along the said sun tracking telescope axis; h. aheat sink cooperating with the said solar pumped laser, positioned inthe said sun tracking telescope radiating heat energy into deep space;i. means for positioning the said laser in the said sun trackingtelescope so that the polarization of the laser beam is perpendicular tothe said axis of rotation of the said first folding mirror andperpendicular to the said sun tracking telescope axis; j. a firstquarter-wavelength plate mounted in the said first gimbals,perpendicular to the spacecraft axis of rotation for changing the saidplane polarized laser beam to a substantially circularly polarized beam;k. a second quarter-wavelength plate mounted in the said spacecraftperpendicular to the said spacecraft axis of rotation for changing thesaid circularly polarized beam to a plane polarized beam; and l. meansresponsive to the said signal to be transmitted to the remote receiver,cooperating with the said plane polarized beam from the said secondquarter-wavelength plate, for modulating the said laser beam.
 5. ThelaseR communication system as claimed in claim 4 wherein a sun seekerdetector is positioned in the said sun tracking telescope and meanscooperates with the said sun seeker for directing the sun trackingtelescope toward the sun.
 6. The laser communication system as claimedin claim 5 wherein the said solar pumped laser is an end pumped laser.